1. Field of the Invention
The present invention relates to a technology for fabricating a transistor, particularly to a method for fabricating a thin-film transistor.
2. Description of the Related Art
In recent years, applications of flat display devices are rapidly developed. Electronics, such as televisions, cell phones, mobiles, and refrigerators, are installed with flat display devices. A thin film transistor (TFT) is a kind of semiconductor devices commonly used in the flat display device, such as a liquid crystal display (LCD), an organic light emitting diode (OLED) display, and an electronic paper (E-paper). The thin film transistor is employed to control voltage and/or current of a pixel of the flat display device for presenting a bright, a dark, or a gray level display effect.
According to different semiconductor materials applied in the thin film transistors, the thin film transistors in current display industries may includes amorphous silicon thin film transistors (a-Si TFTs), poly silicon thin film transistors, and oxide semiconductor thin film transistors. The amorphous silicon thin film transistor is currently the mainstream thin film transistor applied in the display industry because of its mature process techniques and high yield. However, the amorphous silicon thin film transistor may not be good enough to satisfy requirements of foreseeable high performance display devices, because the electrical mobility of the amorphous silicon thin film transistor, which is mainly determined by material properties of amorphous silicon, can not be effectively improved by process tuning or design modification. The typical value of the electrical mobility of the amorphous silicon thin film transistor is smaller than 1 cm2/Vs. On the contrary, the electrical mobility of the poly silicon thin film transistor is much better because of material properties of poly silicon. The typical value of the electrical mobility of the poly silicon thin film transistor is around 100 cm2/Vs. However, because of process issues such as high process complexity and worse uniformity, which is mainly generated by crystallization processes applied to large size substrates, the poly silicon thin film transistors are mainly applied in small size display devices. On the other hand, the oxide semiconductor thin film transistor may be applied for large size substrates without the above-mentioned uniformity issue because the structure of the employed oxide semiconductor material is generally amorphous. The process flexibility of the oxide semiconductor thin film transistor is even better than the amorphous silicon thin film transistor, because the oxide semiconductor material layer may be formed by diverse methods such as sputter depositing, spin-on coating, and inkjet printing. Additionally, the electrical mobility of the oxide semiconductor thin film transistor is generally 10 times larger than the electrical mobility of the amorphous silicon thin film transistor. The typical value of the electrical mobility of the oxide semiconductor thin film transistor is generally between 10 and 50 cm2/Vs. Therefore, the oxide semiconductor thin film transistor is currently the front-runner in the competition of replacing the amorphous silicon thin film transistor in the display industry.
In the traditional technology, as shown in FIG. 1, a gate layer 12 and a gate oxide layer 14 are sequentially formed on a plastic substrate 10, and the gate oxide layer 14 is made of porous silicon oxide. A nickel shadow mask 16 with a pair of holes 17 is used to undertake a patterned process. An ITO layer as a source 18 and a drain 20 is deposited on the gate oxide layer 14 via the holes 17 by a sputtering method. The shadow mask 16 is about 50 μm from the plastic substrate 10, and the shadow mask 16 and the plastic substrate 10 are separated. In the sputtering process, a channel 22 is formed between the source 18 and the drain 20 due to a scattering effect since the shadow mask 16 is a distance from the plastic substrate 10. However, the patterned process of using the shadow mask 16 fails to fabricate the channel with the very short width (only ≧50 μm). Besides, the distance between the shadow mask 16 and the plastic substrate 10 is not precisely controlled taking a consideration of bending of the shadow mask 16 and the plastic substrate 10 themselves, thereby not precisely controlling the uniformity and the size of devices. As a result, the traditional technology is difficult to fabricate circuits.
To overcome the abovementioned problems, the present invention provides a method for fabricating a thin-film transistor, so as to solve the afore-mentioned problems of the prior art.